Bacillus thuringiensis (Bt) is a soil-dwelling Gram-positive bacterium commonly used as a biological pesticide. It is known to produce proteins that are toxic to many insect species, but not to humans or other mammals.

The bacterium generates spores and crystalline proteins (Cry proteins) during sporulation. When ingested by some insects, these Cry proteins are activated in the alkaline conditions of the insect's digestive system, causing it to stop feeding and eventually die.

What it is for and where

Bacillus thuringiensis is used in organic farming and is considered safe for humans and the environment. It is also used in genetic engineering to create crops that produce the toxin Bt and are resistant to certain pests.

Impact on the environment

Bacillus thuringiensis  is generally considered environmentally friendly. It is highly specific to certain pests, so broad-spectrum pesticides are less likely to harm non-target organisms. However, there are concerns about the possibility of pests developing resistance to Bt, especially in the context of Bt crops.

Safety

Bacillus thuringiensis is considered safe for humans, other mammals, birds and fish. Bt toxins are activated by alkaline conditions and specific receptors in the gut, both of which are absent in mammals. However, as with any pesticide, it is important to use Bt responsibly and follow all safety guidelines.

Studies

E. Schnepf, N. Crickmore, J. Van Rie, D. Lereclus, J. Baum, J. Feitelson, D. R. Zeigler, D. H. Dean Bacillus thuringiensis and Its Pesticidal Crystal Proteins  DOI: https://doi.org/10.1128/mmbr.62.3.775-806.1998

Abstract. During the past decade the pesticidal bacterium Bacillus thuringiensis has been the subject of intensive research. These efforts have yielded considerable data about the complex relationships between the structure, mechanism of action, and genetics of the organism’s pesticidal crystal proteins, and a coherent picture of these relationships is beginning to emerge. Other studies have focused on the ecological role of the B. thuringiensis crystal proteins, their performance in agricultural and other natural settings, and the evolution of resistance mechanisms in target pests. Armed with this knowledge base and with the tools of modern biotechnology, researchers are now reporting promising results in engineering more-useful toxins and formulations, in creating transgenic plants that express pesticidal activity, and in constructing integrated management strategies to insure that these products are utilized with maximum efficiency and benefit.

Palma L, Muñoz D, Berry C, Murillo J, Caballero P. Bacillus thuringiensis toxins: an overview of their biocidal activity. Toxins (Basel). 2014 Dec 11;6(12):3296-325. doi: 10.3390/toxins6123296.

Abstract. Bacillus thuringiensis (Bt) is a Gram positive, spore-forming bacterium that synthesizes parasporal crystalline inclusions containing Cry and Cyt proteins, some of which are toxic against a wide range of insect orders, nematodes and human-cancer cells. These toxins have been successfully used as bioinsecticides against caterpillars, beetles, and flies, including mosquitoes and blackflies. Bt also synthesizes insecticidal proteins during the vegetative growth phase, which are subsequently secreted into the growth medium. These proteins are commonly known as vegetative insecticidal proteins (Vips) and hold insecticidal activity against lepidopteran, coleopteran and some homopteran pests. A less well characterized secretory protein with no amino acid similarity to Vip proteins has shown insecticidal activity against coleopteran pests and is termed Sip (secreted insecticidal protein). Bin-like and ETX_MTX2-family proteins (Pfam PF03318), which share amino acid similarities with mosquitocidal binary (Bin) and Mtx2 toxins, respectively, from Lysinibacillus sphaericus, are also produced by some Bt strains. In addition, vast numbers of Bt isolates naturally present in the soil and the phylloplane also synthesize crystal proteins whose biological activity is still unknown. In this review, we provide an updated overview of the known active Bt toxins to date and discuss their activities.